IDO1 and neoplasm: As shown in Figure 2, these strategies comprise those that (1) prevent the recruitment of MDSCs; (2) promote the differentiation of MDSCs into mature cells; (3) deplete MDSCs in the circulation and the tumor; (4) inhibit the elimination of L-arginine mediated by MDSCs; (5) inhibit the activation of IDO in MDSCs; and (6) decrease the levels of ROS and RNS in MDSCs.